With the development of optical devices and the relevant technology, the single-channel rate of the backbone network has been improved greatly. The development of the optical amplifier, dispersion compensation and optical fiber technologies makes the multiplexed wavelengths of Dense Wavelength Division Multiplexing even greater in quantity. The technical development in such two aspects enables the backbone network to gain a considerable progress in its capacity. People at the client side have increasing requirements for information. Users have requirements for even more bandwidth for online games, online movies, downloading from Internet, and Internet Protocol TV (IPTV) that will be available in the near future, which results in a problem of access bottleneck. For solving this problem, many solutions have been proposed such as asymmetrical digital subscriber line, very high rate digital subscriber line, Optical Access Network (OAN). The OAN is classified into active optical network and Passive Optical Network depending on the existence of active devices.
Among the access solutions in the prior art, the PON technology is the most attractive and widely used one with such unique advantages as: 1) PON completely adopts passive devices in its Optical Distribution Network (ODN) to reduce the number of communication stations; 2) PON is easy to be maintained and can be upgraded; and 3) optical transmission which has a high bandwidth is used for access to PON, which can satisfy the current and future demands for bandwidth.
In the prior art, a PON system has such a topological architecture as illustrated in FIG. 1. The PON includes three components: an Optical Line Terminal (OLT) 110, which is installed at a central control station; an Optical Distribution Network (ODN), and a plurality of ONUs 130 (ONU1, ONU2, ONU3, . . . , ONUN, wherein N represents number of the ONUs in the PON), which are installed at the user site. The ODN may include an optical splitter 120 as illustrated in FIG. 1, and also optical fibers. The PON adopts a broadcast mode for the transmission in its downlink, with the downlink information sent from the OLT and received by all the ONUs, each of which extracts downlink information destined to it in accordance with a certain mechanism. In the uplink, the time division multiple access technology is adopted so that multiple ONUs share the uplink bandwidth.
Since the ONUs are physically located at different locations, data frames sent from each of them will arrive at the OLT simultaneously, which may result in transmission collision. To solve the problem, it is necessary to range each ONU so as to prevent the logical time when data frames from each ONU arrive at OLT from colliding with data frames from one another. When ranging an ONU, it is required to reserve for the ranged ONU a certain ranging period during which other ONUs cease sending uplink traffic in order to avoid collision. This period is referred to as ranging window. The OLT periodically sends a ranging grant signal, which carries local time. Upon receiving the grant signal, each ONU temporarily ceases sending uplink traffic. The prior ranging method, by taking the example of performing ranging on ONU1, includes the steps as follows:
1) ONU1 reads the time t1 in a grant signal and writes the time t1 into its local clock on receiving the grant signal sent from OLT.
2) ONU1 sends a response frame which carries its local time.
3) On receiving the response frame, from ONU1, which carries its local time, OLT calculates the difference between the carried time t2 and its local time of its local clock to get the RTT (Round Trip Time), based on which the ranging is performed.
The following issues are found in developing the present invention:
When performing ranging on the ONU, the windowing duration is necessary to cover the entire system and the transmission time corresponding to the distance from the optical splitter to the OLT. Consequently, one ONU has to be provided with a large data memory to avoid traffic loss due to the long ranging distance and the long ranging time. In addition, other ONUs may not transmit traffic respectively before the ranged ONU completes the ranging, which results in that a large quantity of bandwidth is occupied. Furthermore, it takes a plenty of time to perform the ranging which requires optical-to-electrical and electrical-to-optical conversions. Moreover, the standard maximum RTT is adopted as the ranging time, thereby introducing a long delay.
The processing time of ONU needs redefining when the system speeds up. The ranging method lacks robustness. It is required to redefine a balance time when the system distance exceeds the standard maximum distance. During the ranging, such processing steps as reading an OLT clock tag and writing into the ONU clock tag exist at the ONU side, which introduces a delay and makes the system more complex.